1. Field of the Invention
This invention relates to a vehicle automatic transmission control system and more particularly to a vehicle automatic transmission control system which applies fuzzy logic to enable control resembling that based the judgments and operations of an expert driver in the case of a conventional manually shifted vehicle transmission.
2. Description of the Prior Art
In driving a vehicle equipped with a conventional manually shifted transmission, the driver judges the appropriate time or instant for changing gears on the basis of his observation of the surrounding circumstances and the vehicle operating state and then, when has determined that the time is right, operates the clutch pedal and the gearshift lever to shift the transmission to what he has decided is the appropriate gear. This is a fairly troublesome operation, especially because it has to be done quite frequently, and to make it unnecessary there has been developed the automatic transmission. Today, the majority of new vehicles sold are equipped with automatic transmissions. The control system for one type of automatic transmission includes a shift valve as one member of a hydraulic circuit. To one side of this valve there is applied a throttle pressure proportional to the degree of opening of the throttle and to the other side there is applied a governor pressure proportional to the vehicle speed, and gear changing is carried out automatically by supplying/cutting off hydraulic pressure to/from a gear clutch in accordance with the ratio between the two pressures. More recently, this type of control has been replaced with an electronic control system employing a microcomputer which has a gearshift program stored in a memory thereof. In this system, the degree of throttle opening and the vehicle speed are used as address data for retrieving the proper time for changing gears from the gearshift program and then a solenoid valve is energized/de-energized accordingly to drive the shift valve so as to cause it to shift the transmission.
In the conventional automatic transmission the time for changing gears is determined solely on the basis of the degree of throttle opening and the vehicle speed instead of in accordance with the judgment and operations of the driver himself. This inevitably causes unnatural gear changing. For example, when the vehicle is running up hill and the driver returns the throttle opening to that for cruising on level ground, the transmission will shift up, depending on the vehicle speed at the time. As a result, the amount of reserve power available will become insufficient, making it necessary for the driver to press down on the accelerator pedal again, which will cause the transmission to shift down. This shiftup, shiftdown cycle will repeat itself, giving the driver the impression of being busy. This same problem also arises in such cases as when the vehicle is used to pull a camper, when the weight of the vehicle changes because of a change in the weight of the load or the like, or when the engine charging efficiency declines during driving at high altitudes.
The answer to why the driver presses down on the accelerator pedal and thus opens the throttle valve is simply that he wants to accelerate and expects that when he depresses the accelerator pedal further the vehicle will respond to his desire. Stated in another way, the reason that the aforesaid problem arises is that the control system issues a gearshift command notwithstanding that the vehicle has been deprived of sufficient controllability by a decrease in reserve power. Thus, the problem arises because the control system does not do what it should do, namely, to accurately ascertain the motive force and the driving resistance and shift up only after confirming that the motive force is greater than the driving resistance, i.e. after confirming that there is reserve force available.
A way for overcoming this problem was recently proposed in Japanese Laid-open Patent Publication No. 60(1985)-143133. According to the proposed technique, the amount of torque desired by the driver is determined from the amount of depression of the accelerator pedal and the desired acceleration is calculated by subtraction of the pre-calculated climbing resistance. Further, the one among a plurality of pre-prepared optimum fuel-economy gearshift diagrams which is best matched to the calculated climbing resistance is selected, the degree of opening of the throttle is controlled on the basis of data obtained from the constant acceleration locus on the selected gearshift diagram, and the new degree of throttle opening and the vehicle speed are used as address data for determining from the gearshift diagram what gearshift operation should be carried out for maintaining the acceleration prior to the change in the degree of throttle opening.
With this prior art method, however, while the gearshift judgment is made with consideration being given to the torque desired by the driver, it is after all made solely on the basis of a predetermined gearshift diagram and thus is able to respond only to preset conditions. What is more, as in the earlier controls, this technique also determines the gearshift timing solely on the basis of the degree of throttle opening and the vehicle speed and in this point it has the same shortcomings as the earlier controls.
In other words, if the vehicle were one with a manually shifted transmission, the driver would have been aware that he was driving up hill and avoided making an improper shiftup at the time of changing gears. That is, because he has a grasp of the surrounding circumstances and other aspects of the vehicle operation, the driver of the vehicle with the manually shifted transmission would be aware of a magnitude of the motive force and would also anticipate the increase/decrease in the tractive force apt to result if he should shift gears. He would thus decide when to shift gears by selection from among the various rules he has learned through his driving experience. What this means is that the aforesaid inconvenience of the conventional control arises because the judgments and operations of the human driver are ignored, i.e. not reflected in the control. More specifically, in the conventional vehicle gearshift control systems the gearshift timing is, as a basic principle, decided mechanically from the degree of throttle opening and the vehicle speed, and thus is not capable of determining this timing on the basis of a large number of vehicle operation variables. As a result, the aforesaid problems cannot be avoided.
What is more, the situation described in the foregoing is true not only of multi-step transmission but also of continuously variable transmissions. This is because the continuously variable transmission also varies the speed ratio (gear ratio) according to the running state of the vehicle and in this point is no different than the multi-step transmission. In this specification, therefore, the term "gearshift" is sometimes used to mean both changing of gear position in a multi-step transmission and the changing of the speed ratio in the continuously variable transmission.